Multiple-description coding (MDC) describes a source coding technology used for information transmission over an unreliable network. This technology generates at least one transmission bit stream and incorporates redundancy into each bit stream without additional delay, thus providing a source coding algorithm to prevent packet loss. The MDC technology represents source information through multiple descriptions, each of which describes the proximity of source information. When obtaining information from each other, a number of descriptions can generate the best proximity to the source information.
MDC is available in many methods, such as parity selection, dual transformation, and scalar quantization. 1. The multiple-description parity selection separates code parameters, namely, time domains or frequency domains, into odd numbers and even numbers in accordance with their indexes or natural orders, and then encodes those odd and even numbers. With different odd and even numbers, two irrelevant descriptions have no redundancy between them, and will not increase the overall coding rate. The multiple-description dual transformation maps the parameters, namely, time or frequency domains, into two descriptions through transform matrix. The dual transformation matrix can control the dependence between these two descriptions. Larger dependence causes more bits needed for coding and a higher coding rate. If one description is lost, the signal restored by another description provides higher quality. The multiple-description scalar quantization selects a scalar quantizer with smaller precision to quantize the parameters, namely, time or frequency domains. The higher precision causes greater dependence between two descriptions.
During Internet traffic peaks, the packet loss ratio reaches as high as 60%. The loss of one, two or even three packets accounts for a large percentage of total packet loss. The percentage varies according to Internet congestion. Higher congestion usually causes a larger packet loss. When the Internet is heavily congested, one-packet loss accounts for about 60% of the total loss, two-packet loss about 25%, and three-packet loss about 8%. Among various Internet multimedia applications, such as real-time video-audio communication and video on demand (VOD), both video and audio encoders must resist against three-packet loss to ensure video and voice quality. Two-description coding, however, can just resist against one-packet loss, and three-description coding against two-packet loss simultaneously. Therefore, the four-description coding is implemented to resist against three-packet loss.
In the prior art, four-description coding provides a single-type multiple-description method. For example, descriptions M1, M2, M3, and M4 have the same quantization precision and ¼ quantization interval between each two. If a multi-description decoder receives these descriptions and combines them through multiple-description decoding (MDD), this decoder can quadruple the quantization precision. Such precision, however, will be reduced if one or more descriptions are lost. Four-description coding involves 15 packet loss scenarios, including one for all four descriptions received, four for the loss of one description, six for the loss of two descriptions, and four for the loss of three descriptions. Therefore, decoding must be implemented for each of the 15 packet loss scenarios.
While studying and practicing the prior art, the inventor has identified the following drawbacks: If the packet loss rate reaches 50% and the lost quantization precision exceeds 50%, more quantization errors arise when decoding is performed through the received descriptions, thus degrading user experience.